Make SQLAlchemy optional for Postgres provider

[mohit7705]

This PR makes SQLAlchemy an optional dependency for the Postgres provider,
following the same pattern used by other providers (e.g. Presto, Exasol).

Summary of changes:

• Removed hard SQLAlchemy imports from module level
• Imported SQLAlchemy lazily inside methods that require it
• Raised AirflowOptionalProviderFeatureException when SQLAlchemy is missing
• Kept SQLAlchemy as an optional extra dependency

This aligns the Postgres provider with the guidance described in the parent issue.

[mohit7705]

hi @potiuk , @Prab-27 , @justinpzkad

The changes are ready now. Could you please approve the workflows so CI can start running on this PR?

Thanks a lot!

[mohit7705]

hi @vincbeck
can you pls verify one more time ,

[mohit7705]

@vincbeck ,
can you pls tell me what to do next?

[vincbeck]

Looks good on my side + CI is the passing. I'll wait others to approve as well just to be sure

[mohit7705]

can you pls tag them

[vincbeck]

You already did, you need to give them time. Open source works thanks to people willing to contribute on their free time. They'll review your PR at some point, do not worry

[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.